The Commissioning Instrument was installed at the DESI focal plane behind the DESI Corrector during Spring 2019. Its focal surface consisted of five CCD sensors and 22 fiber-illuminated fiducials whose positions were engineered to work effectively with Platemaker. The surface was enclosed in a tall steel cylinder. The entire Commissioning Instrument was designed to have the same mass and mass distribution as DESI’s Focal Plane System to ensure realistic operations.
The Commissioning Instrument was installed at the Mayall Telescope and began a two-month commissioning run on April 1 2019. During Summer 2019, it was removed to make way for the installation of the DESI Focal Plane System.